1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display device having a plurality of thin film transistor driver elements and a liquid crystal member for displaying an image.
2. Description of the Related Art
A liquid crystal display (LCD) device, which has high density and large capacities, and which is capable of displaying an image at high speed with high resolution, is now widely used.
An active matrix type LCD (AM-LCD) device having thin film transistors (TFTS) as control elements are frequently used for the following reasons.
A display having a high contrast can be obtained without any crosstalk between adjacent pixels. Also, a transparent type display can be carried out, and a display area can be easily increased.
In the transparent type AM-LCD using an active matrix substrate (hereinafter called AM-plate) having TFTs as driver elements, a gate-insulating layer was formed on the AM-plate. A plurality of pixel electrodes were formed on the gate-insulating layer by photolithography. Moreover, a plurality of signal electrodes (signal lines) were formed on the gate-insulating layer to be on the same surface, the pixel electrodes having been formed by photolithography. Due to this, to compensate for a shift of a plurality of patterns formed on a photo resist layer at a pattern forming time, each outer periphery of the pixel electrodes was formed to be inside by 2 to 4 .mu.m from each TFT and each line.
Particularly, there was a problem in that a pixel electrode voltage was leaked to the signal line due to increase in coupled capacitance between the signal electrode (signal line) and the pixel electrode. As a result, display definition was decreased. To prevent decrease in display definition, each of the outer periphery of the pixel electrode was formed to be several mm more inside from the above-mentioned state.
However, there occurred another problem in which an area for a pixel region was reduced, and an aperture ratio of the active matrix substrate, that is, the liquid display device was reduced.
Moreover, in the conventional case, to prevent a contrast ratio from being reduced by light leaking from the pixel electrode of the active matrix substrate, a black mask was formed on an opposite substrate. However, an accuracy of the position between the pixel electrode and the black mask depends on an assembly accuracy of an LCD cell (AM-plate and the opposite substrate are assembled). Then, a width of the black mask is largely set to allow an assembly accuracy of 6 to 10 .mu.m. As a result, there is a problem in which the aperture ratio of the liquid crystal display device is further reduced if the above-mentioned structure is used.
For preventing display failure caused by a connection failure between a source electrode of TFT and the pixel electrode, a portion where both electrodes overlap with each other must be increased. However, in the conventional case, the source electrode is formed of transparent metal. Due to this, if the area of the overlapping portion is increased to make a sufficient contact between both electrodes, the aperture ratio is further decreased.
An amount of light transmission is decreased by the above-mentioned reduction of the aperture ratio, and luminance of the displayed screen is reduced. Then, if the luminance (amount of light) of a back light lamp is increased to ensure a predetermined luminance, power consumption is increased.
Moreover, in view of the structure of the above-mentioned active matrix substrate, there is the possibility in each of the pixel electrodes and each of the signal lines, which are formed on the same surface, of a short-circuit. For this reason, there is a problem in the occurrence of display defects, where "black points on the LCD cell do not disappear." That is, the pixel, which is positioned at a portion where the short-circuit occurs, maintains an ON state.